Until now, the Swedish prosecutor, Marianne Ny, had always said Mr Assange was only wanted for questioning.

Geoffrey Robertson, for Mr Assange, told Judge Howard Riddle, the chief magistrate for Westminster, that the extradition request must be turned down because his client would not receive a fair trial in Sweden.

He said: "It is likely that he will be tried in Sweden in secret, behind closed doors, in a flagrant denial of justice.

"Given the amount of vilification that he has been subjected to, it's obviously unfair that he should then be ushered into a secret court and convicted or even acquitted.

In the public gallery Tony Benn, Bianca Jagger and Jemima Khan were among those who came to lend their support. Miss Khan, who has previously offered sureties to guarantee Assange's bail, was asked to remove her black leather boots during security checks.

Mr Assange's appearance sparked a media frenzy. In front of the courthouse, more than 100 cameramen and photographers were penned in by temporary metal fencing. All 100 media seats, split between Court 3 and a temporary annexe outside, were taken weeks ago.

Mr Assange was held in December under a European Arrest Warrant (EAW) and spent several days in prison before being released on conditional bail. Since then he has been living under effective house arrest at a Norfolk manor house owned by a friend.

His legal team had planned argue that the EAW does not apply in cases where someone is only wanted for questioning, but the Swedish prosecutor's change of approach may make that defence irrelevant.

“You will see some pretty fundamental challenges to the European Arrest Warrant scheme.

“The EAW is meant for people who have been charged and are facing trial. There is a separate scheme, which should be used in cases like this, where someone is wanted for questioning.

“The fact that this isn’t happening is something I find really worrying.”

The defence will also claim Mr Assange's life will be in danger if he is extradited to Sweden. They will say that he could face “rendition” from there to the US, where there have been calls for his arrest on national security grounds following WikiLeaks’ publication of thousands of leaked US Embassy cables.

An outline of the defence case already published by his legal team states: “There is a real risk that, if extradited to Sweden, the US will seek his extradition and/or illegal rendition to the USA, where there will be a real risk of him being detained at Guantánamo Bay or elsewhere … there is a real risk that he could be made subject to the death penalty.”

Mr Assange’s lawyers will argue that extradition to Sweden would breach his human rights because Sweden has, in the past, extradited suspects to Egypt, which has been accused of torture.

Mike Huckabee, a potential Republican candidate for the 2012 presidential election, has suggested the people responsible for leaking US diplomatic cables should be executed, while Sarah Palin, the former Republican vice-presidential candidate, said Mr Assange “should be hunted down just like al-Qaeda”.

Mr Assange was accused of assaulting two women, Miss A and Miss W, on separate occasions during a stay in Sweden last year and raping one of the women. He was questioned by police and denied the allegations, saying he had consensual sex with both women.